Page 169 - Origin and Prediction of Abnormal Formation Pressures

144 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.
6000
6000 !

-
7000 - .~_.. 7000 -
-4===
8000 -
Ib/gal
8000 -
c~
Q) 9000 ~'~ II ~ 9000 [9"5/8" casin~ ----~
r~ ~ 9-5/8" casing l 12.5 -
m l
c~ I0,000 - ~:- II :~ I0'000- l
0 16.5 -
l
11,000 -
t
(D
12,000 7" casing ~~'~ i 12,000 " casi t t 16.4 -
t
13,000 - 13,000 -
~5" casing i "oa 'ng 17.6 -
14,000 14,000 -
17.9 -
15,000 i -- 15,000 I I J J, , I
0.1 1 2 60 50 40 30 20 10 0
Shale Resistivity, Sigma Shale, Y sh

Fig. 5-12. Plots of a shale resistivity and Z-shale values versus the true vertical depth, which define the
overpressure environment in offshore U.S. Gulf Coast well. (Modified after Taneja and Carroll, 1985.)

by side-tracking the original well above the casing restriction and redrilling to this
target.
The mud weight required to safely drill this well initially in 1946 to the Homeseekers
'A' target zone was approximately 14.0 lb/gal, because both the sands and shales
contained overpressures equivalent to this specific weight of drilling mud. Due to
production-related pressure depletion over the years, however, most of the sands had
exhibited pressure gradients throughout this oilfield considerably less than hydrostatic.
Thus, it was known that the interval to be redrilled would not sustain nor require the
high mud weights used to drill the original well.
The PNC log was run in the old cased wellbore to evaluate present pressure
conditions. Fig. 5-11B presents the Z-trend versus depth to a depth of 9110 ft. The
maximum mud weight determined to reach the Homeseekers 'A' sands was 10.2 lb/gal.
Fig. 5-11C clearly shows the change in shale pressure due to pressure depletion of the
sands. The well was side-tracked at 8120 ft, and redrilled to 9215 ft without difficulty.
A mud weight of 10.4 lb/gal was required for redrilling the well rather than the mud
weight (MW) of 14.3 lb/gal used for the original drilling fluid.
Plots of shale resistivity and Z-shale values versus true vertical depth (Fig. 5-12),
were used to define the overpressure environment for 18 offshore Gulf Coast wells in
12 fields to establish the generalized compaction trend (Taneja and Carroll, 1985). The